A study was made of the composition of the liquid phase formed upon absorption of nitrogen dioxide by sulfuric acid as a function of the concentration of oxides in the gas, temperature and concentration of sulfuric acid. It was found that in the process of absorption of NO2, mixtures consisting of HNO3 HNOSO4 H2SO4 H2O are formed, containing quite definite maximum concentrations of HNOSO4, dependent on temperature, NO2 concentration in the gas and concentration of absorbing sulfuric acid. This dependence was established. An equation was produced for determination of the maximum concentration of HNOSO4. The possibility is shown of producing mixtures with high HNO3 content by absorption of nitrogen oxides.